Nftables

What is nftables?

nftables is the successor to iptables. It replaces the existing iptables, ip6tables, arptables and ebtables framework. It uses the Linux kernel and a new userspace utility called nft. nftables provides a compatibility layer for the ip(6)tables and framework.

Introduction

As with the iptables framework, nftables is build upon rules which specify the actions. These rules are attached to chains. A chain can contain a collection of rules and is registered into the netfilter hooks. Chains are stored inside tables. A table is specific for one of the layer 3 protocols. One of the main differences with iptables is that there are no predefined tables and chains anymore.

Tables

A table is nothing more than a container for your chains. With nftables there are no predefined tables (filter, raw, mangle...) anymore. You are free to recreate the iptables-like structure, but anything might do.
Currently there are 5 different families of tables:

ip: Used for IPv4 related chains;

ip6: Used for IPv6 related chains;

arp: Used for ARP related chains;

bridge: Used for bridging related chains;

inet: Mixed ipv4/ipv6 chains (kernel 3.14 and up).

It is not hard to recognize the old tables framework in these tables. The only new one is the inet table which is used for both IPv4 and IPv6 traffic. It should make firewalling for dual-stack hosts easier by combining the rules for IPv4 and IPv6.

Chains

Chains are used to group together rules. As with the tables, nftables does not have any predefined chains. Chains are grouped in base and non-base types. Base chains are registered in one of the netfilter hooks. A base chain has a hook its registered with, a type and a priority. Non-base chains are not attached to a hook and they don't see any traffic by default. They can be used to arrange a rule-set in a tree of chains.
There are currently three types of chains:

filter: for filtering packets

route: for rerouting packets

nat: for performing Network Address Translation. Only the first packet of a flow hits this chain, making it impossible to use it for filtering.

The hooks that can be used are:

prerouting: This is before the routing decision, all packets entering the machine hits this chain

input: All packets for the local system hits this hook

forward: Packets not for the local system, those that need to be forwarded hits this hook

output: Packets that originate from the local system pass this hook

postrouting: This hook is after the routing decision, all packets leaving the machine hits this chain

Note

The ARP address family only supports the input and output hook

Note

The bridge address family only seems to supports the input, forward and output hook

Priorities

Note

Priorities do not currently appear to have any effect on which chain sees packets first.

Note

Since the priority seems to be an unsigned integer, negative priorities will be converted into very high priorities.

Rules

Rules specify which action has to be taken for which packets. Rules are attached to chains. Each rule can has an expression to match packets with and one or multiple actions when matching. Main differences with iptables is that it is possible to specify multiple actions and that by default counters are off. It must be specified explicitly in rules if you want packet- and byte-counters for a rule.
Each rule has a unique handle number by which it can be distinguished.
The following matches are available:

ip: IP protocol

ip6: IPv6 protocol

tcp: TCP protocol

udp: UDP protocol

udplite: UDP-lite protocol

sctp: SCTP protocol

dccp: DCCP protocol

ah: Authentication headers

esp: Encrypted security payload headers

ipcomp: IPcomp headers

icmp: icmp protocol

icmpv6: icmpv6 protocol

ct: Connection tracking

meta: meta properties such as interfaces

Matches

Match

Arguments

Description/Example

ip

version

Ip Header version

hdrlength

IP header length

tos

Type of Service

length

Total packet length

id

IP ID

frag-off

Fragmentation offset

ttl

Time to live

protocol

Upper layer protocol

checksum

IP header checksum

saddr

Source address

daddr

Destination address

ip6

version

IP header version

priority

flowlabel

Flow label

length

Payload length

nexthdr

Next header type (Upper layer protocol number)

hoplimit

Hop limit

saddr

Source Address

daddr

Destination Address

tcp

sport

Source port

dport

Destination port

sequence

Sequence number

ackseq

Acknowledgement number

doff

Data offset

flags

TCP flags

window

Window

checksum

Checksum

urgptr

Urgent pointer

udp

sport

Source port

dport

destination port

length

Total packet length

checksum

Checksum

udplite

sport

Source port

dport

destination port

cscov

Checksum coverage

checksum

Checksum

sctp

sport

Source port

dport

destination port

vtag

Verification tag

checksum

Checksum

dccp

sport

Source port

dport

destination port

ah

nexthdr

Next header protocol (Upper layer protocol)

hdrlength

AH header length

spi

Security Parameter Index

sequence

Sequence Number

esp

spi

Security Parameter Index

sequence

Sequence Number

ipcomp

nexthdr

Next header protocol (Upper layer protocol)

flags

Flags

cfi

Compression Parameter Index

icmp

type

icmp packet type

icmpv6

type

icmpv6 packet type

ct

state

State of the connection

direction

Direction of the packet relative to the connection

status

Status of the connection

mark

Connection mark

expiration

Connection expiration time

helper

Helper associated with the connection

l3proto

Layer 3 protocol of the connection

saddr

Source address of the connection for the given direction

daddr

Destination address of the connection for the given direction

protocol

Layer 4 protocol of the connection for the given direction

proto-src

Layer 4 protocol source for the given direction

proto-dst

Layer 4 protocol destination for the given direction

meta

length

Length of the packet in bytes: meta length > 1000

protocol

ethertype protocol: meta protocol vlan

priority

TC packet priority

mark

Packet mark

iif

Input interface index

iifname

Input interface name

iiftype

Input interface type

oif

Output interface index

oifname

Output interface name

oiftype

Output interface hardware type

skuid

UID associated with originating socket

skgid

GID associated with originating socket

rtclassid

Routing realm

Statements

Statements represent the action to be performed when the rule matches. They exist in two kinds: Terminal statements, unconditionally terminate the evaluation of the current rules and non-terminal statements that either conditionally or never terminate the current rules. There can be an arbitrary amount of non-terminal statements, but there must be only a single terminal statement.
The terminal statements can be:

accept: Accept the packet and stop the ruleset evaluation.

drop: Drop the packet and stop the ruleset evaluation.

reject: Reject the packet with an icmp message

queue: Queue the packet to userspace and stop the ruleset evaluation.

continue:

return: Return from the current chain and continue at the next rule of the last chain. In a base chain it is equivalent to accept

jump <chain>: Continue at the first rule of <chain>. It will continue at the next rule after a return statement is issued

goto <chain>: Similar to jump, but after the new chain the evaluation will continue at the last chain instead of the one containing the goto statement